In recent years, as a method for forming solder resist layers on printed wiring boards for consumer use and industrial use, a developable solder resist composition with excellent resolution and dimensional accuracy has been widely used, instead of a screen printing method, in order to increase the density of wiring on the printed wiring board.
Additionally, in recent years, there is an increasing use of optical elements, such as light-emitting diodes, mounted directly on a printed wiring board covered with a solder resist layer, wherein the light-emitting diodes are often used for: backlights in liquid crystal displays for mobile terminals, personal computers, and televisions; and light sources for lighting devices. Furthermore, when titanium dioxide is contained in the solder resist layer on the printed wiring board mounted with optical elements, the solder resist layer is whitened and therefore light emitted from the optical elements is efficiently reflected at the solder resist layer (see JP5513965B2).
However, in a process of curing the solder resist composition under exposure to light, titanium dioxide contained in the solder resist composition may cause difficulty in curing of the solder resist composition due to titanium dioxide reflecting or absorbing light. Especially when the solder resist composition contains a large amount of titanium dioxide, it is difficult for the solder resist layer formed with the solder resist composition to completely cure down to a deep part of the solder resist layer. If the deep part of the solder resist layer is not thoroughly cured, it is likely for defects to occur, such as lowered resolution in development, wrinkles on the solder resist layer caused by a difference in curing shrinkages of the deep part and a surface of the solder resist layer, and cracks on the solder resist layer when heated due to a partial stress caused by a difference in thermal expansion coefficients of the deep part and the surface of the solder resist layer. In addition, the solder resist composition is also required to have good storage stability.